In FIG. 1, there is shown an example of a previously proposed water purification device equipped with a separation tank. In FIG. 1, numeral 1 denotes a filter tank, only the upper part of which is shown, and in the lower part of which is provided granular filter material mainly consisting of filter sand, not visible in the drawing, and in the upper part of which is provided a separation tank 2. The separation tank 2 is cylindrical and has a laterally directed inlet 3 at the upper part thereof through which water discharged from tank 1 is supplied during washing. The lower part of the tank 2 has the shape of a hopper and is provided with an outlet 4 for granular filter material. A cover plate 5 is mounted on the outlet 4 in an inclined position for opening and closure as desired. In the upper wall of the separation tank 2, there is mounted a water outlet 6a corresponding to the water inlet 3. The outlet 6a is connected to an external water drain pipe 6.
During washing of the granular filter material, the washing water introduced under pressure into filtration tank 1 flows into separation tank 2 via water inlet 3 so as to cause a vortex flow 7a of water along the inner wall, the water having granular filter material from tank 1 entrained therein. The vortex flow 7a travels spirally downwardly along the inner wall of the separation tank 2. The granular filter material entrained in the water is separated centrifugally and deposited along the inner lower wall of the tank 2. The cover plate 5, which is normally closed during washing under the effect of buoyancy, is opened under the weight of the accumulated filter material, so that the filter material is returned to the lower portion of the filter tank 1. The direction of flow of the discharged washing water is reversed at the bottom of tank 2 and it rises through the tank 2 centrally as tornado flow 7b so as to be discharged to the outside through water outlet 6a and water drain pipe 6.
In the conventional water purification device of this kind, contaminated water resulting from the washing operation is discharged to the outside as a tornado flow 7b in the separation tank 2, so that heaped filter material may be entrained in the tornado flow and discharged to the outside with this washing water.